The achievement of low-loss (less than 2 dB/km) transmission has made the optical fiber waveguide the leading contender as the transmission medium for a variety of future systems ranging in length from several meters to kilometers. This new low-loss medium offers the potential of many significant advantages compared with metallic conductors, including: long distance transmission without repeators, immunity from electromagnetic interference, cross-talk and ground loop, high bandwidth capabilities, small size and weight, high degree of intercept security and dielectric isolation, and long term cost reduction. The foregoing desirable features of optical fiber waveguides have strongly stimulated efforts in the supporting technologies such as fiber preparation and cabling, long life solid state sources and high performance receivers. As a result of the considerable progress in these areas, the utilization of optical fibers in military systems appears imminent.
There are currently strong trends in military system design, particularly shipboard and airborne, toward micro-miniaturization, digital processing, and system level integration in order to achieve smaller size, weight, and power consumption along with lower cost and improved reliability. Since these trends naturally point to data bus multiplexing it is imperative that consideration be given to fiber optic data distribution systems. There are currently two fiber optic configurations being considered for the distribution of data to a set of remote terminals. One is a serial distribution system that employs T access couplers, and the other is a parallel system employing a Star coupler. To date, both of these systems have been demonstrated using fiber optic bundles rather than single optical fibers. The compelling reason for the use of bundles to this point is that couplers for both T and Star single strand systems have not been developed. Both 1 dB insertion loss cable connectors and 7 dB insertion loss star couplers suitable for bundle systems have, however, been reported in the open literature. See, for example, an article by Frank L. Thiel, Roy E. Love, and Rex L. Smith in the "Journal of Applied Optics", Volume 13, page 240 (1974). See also an article by M. C. Hudson and F. L. Thiel, "Journal of Applied Optics", Volume 13, page 2540 (1974). Bi-directional access couplers with a 1 dB insertion loss, however, have not heretofore been reported.
It can be shown that generally the signal level advantage of the Star format over the serial format increases as the number of terminals in the system increases. The advantage is more pronounced the higher the insertion loss of both the cable connectors and the access couplers. It is, however, relatively insensitive to the insertion loss of the Star coupler. This fact affects the practicality of a serial distribution system assembled with fiber bundles rather than with a single fiber, since in this case the insertion losses of both the cable connectors and access couplers must include the packing fraction loss of the bundled fibers which at best is on the order of 1 dB. However, since systems which employ single fibers as a communication channel do not suffer from the packing fraction problem, both the cable connectors and the access couplers can, in principle, be fabricated with extremely low insertion losses. Give such low-loss connectors and couplers, it can be shown that the serial distribution format for a single strand fiber system can be assembled to serve a modest number of remote terminals without consuming an unreasonable portion of the available power budget from normally used sending and receiving equipments. It can also be shown that for a modest number of terminals, say 20 or less, the power budget advantage of the star system is not excessively large. The cost of the single fiber serial system, where it can meet systems requirements, is obviously less than that of either of the bundled type systems or of a single fiber star system since the amount of optical waveguide cable being used is significantly less.
Such a system depends, however, upon the availability of a low-loss direct access coupler to a single fiber which has not heretofore been available. An indirect access coupler for selective mode coupling has been disclosed in the copending patent application of Michael K. Barnoski and Viktor Evtuhov entitled, "Angle Selective Coupler for Optical Fiber", filed Aug. 4, 1975 as Ser. No. 601,863 and assigned to the same assignee as is the present application. Such an indirect coupler, however, is not intended for use in systems of the type considered above, which require a direct access coupler.